1. Field of the Invention
The present invention relates to silicas having an inhomogeneous structure or composition, to processes for preparing them, and to their use as carrier material.
2. Description of the Background
Readily dispersible silicas are prepared, for example, by the procedure described, for instance, in EP 0 901 986 or EP 0 647 591 by precipitating waterglass with sulfuric acid, followed by drying. The dried products are subsequently ground and/or granulated.
By means of mechanical granulation, any silica can be prepared in dust-free form;
however, this additional process step generally brings about a deterioration in the dispersibility of the silica.
In another process, silicas are prepared, likewise by acid precipitation, but are dried by spraying with hot air and at the same time are shaped into beads, which are easily destroyed. Thus EP 018 866 describes the preparation of spray-dried silica having an average particle diameter of more than 80 μm, the particles being solid and possessing a homogeneous structure.
Spray-dried silicas as described in EP 0 018 866 are particularly suitable as carrier materials, because they are dust-free and possess a high absorbency. The ability to operate without dust being generated is an important criterion for the processing of the silica, because simple processing of the silicas without the need for suction exhaust units is of great economic importance. Besides freedom from dust, the specific surface areas (BET, CTAB) and the oil absorption capacity (DBP) are important for carrier material utility.
In contrast to mechanical granulation, spray drying cannot be used to prepare all silicas in dust-free form.
One type of silica generally does not meet all of the required criteria. Mixtures of two or more types of silica can frequently be prepared, but such mixtures normally generate excessive amounts of dust. A need therefore continues to exist for a silica which at one and the same time covers broad ranges of physicochemical data such as BET or CTAB surface area, and which has good absorbency and generates only low quantities of dust. As already stated, this cannot be achieved for all silicas by means of spray drying or granulation.